This invention relates to the production of superplastically formed panel structures, and is more particularly directed to a process for superplastic forming of a plurality, e.g. of three, sheets, namely, a core sheet and two face sheets joined together as by welding into a panel structure, without formation of wrinkles or grooves in the face sheets at the points of attachment, e.g. welds, with the core sheet.
When a face sheet of a superplastic forming pack is joined to a core sheet by diffusion bonding, welding, and the like, followed by superplastic forming to produce a panel structure in conventional practice, face sheet grooves or wrinkles, commonly called "eye brow", occurs during forming caused by core sheet pulling forces. This is illustrated in FIG. 1 of the drawing. Thus, during superplastic forming of the forming pack 10 comprised of face sheets 12 and 14 and core sheet 16, the forming pressurization at P.sub.1 generates an excessive face sheet pulling force F on core sheet 16 which results in a core panel 18, as seen in FIG. 2, having undesirable grooves 20 in the face sheets 12 and 14 at their points of attachment, namely welds 22, with the core sheet 16, forming the web 24 of the panel structure. Once the face sheets stretch during superplastic forming, they cannot be contracted or shrunk to avoid this problem.
U.S. Pat. No. 4,811,890 to Dowling et al discloses a process for eliminating core distortion in diffusion bonded and superplastically formed structures wherein following superplastic forming and release of the internal gas pressure and during cooling of the metal panel structure and while the metal panel is still at a temperature where the metal remains flexible, a low internal differential pressure is introduced into the interior of the panel, expanding the panel out to its original form against the die as a restraint, and straightening the thin core cell walls of the panel which were distorted following superplastic forming.
However, the above patent does not address the problem of avoiding wrinkles or grooves in the face sheets of a superplastically formed core panel formed of a core sheet bonded as by diffusion bonding or welding to face sheets, at the bonded areas of the face sheets and the core sheet.
It is an object of the present invention to provide a process for eliminating face sheet wrinkles or grooves in a core panel formed during superplastic forming of the core panel.
Another object is the provision of simplified procedure including means in conjunction with a superplastic forming pack comprised of a core sheet bonded, as by welding or the like, to a pair of face sheets, to prevent grooves or wrinkles in the face sheets at the bonded areas of the core sheet with the face sheets, caused by core sheet pulling forces during superplastic forming.
Still another object is to provide a process of the above type wherein the core sheet and face sheets of the superplastic forming pack are comprised of titanium or aluminum, or other superplastic materials.
Yet another object is the provision of an improved core panel or structure produced by the above process.
Other objects and advantages of the invention will appear hereinafter.